Separation of n-propyl acetate from n-propanol and water by extractive distillation

ABSTRACT

n-Propyl acetate cannot be completely removed from n-propyl acetate - n-propanol - water mixtures by distillation because of the presence of the minimum ternary azeotrope. n-Propyl acetate can be readily removed from mixtures containing it, n-propanol and water by using extractive distillation in which the extractive distillation agent is a higher boiling oxygenated or nitrogenous organic compound or a mixture of these. Typical examples of effective agents are N-methylpyrrolidone; triethanolamine; N-methylpyrrolidone and ethylene glycol.

FIELD OF THE INVENTION

This invention relates to a method for separating n-propyl acetate fromn-propanol using certain higher boiling liquids as the extractive agentin extractive distillation.

DESCRIPTION OF PRIOR ART

Extractive distillation is the method of separating close boilingcompounds or azeotropes by carrying out the distillation in a multiplaterectification column in the presence of an added liquid or liquidmixture, said liquid(s) having a boiling point higher than the compoundsbeing separated. The extractive agent is introduced near the top of thecolumn and flows downward until it reaches the stillpot or reboiler. Itspresence on each plate of the rectification column alters the relativevolatility of the close boiling compounds in a direction to make theseparation on each plate greater and thus require either fewer plates toeffect the same separation or make possible a greater degree ofseparation with the same number of plates. When the compounds to beseparated normally form an azeotrope, the proper agents will cause themto boil separately during extractive distillation and thus make possiblea separation in a rectification column that cannot be done at all whenno agent is present. The extractive agent should boil higher than any ofthe close boiling liquids being separated and not form minimumazeotropes with them. Usually the extractive agent is introduced a fewplates from the top of the column to insure that none of the extractiveagent is carried over with the lowest boiling component. This usuallyrequires that the extractive agent boil twenty Centigrade degrees ormore higher than the lowest boiling component.

At the bottom of a continuous column, the less volatile components ofthe close boiling mixtures and the extractive agent are continuouslyremoved from the column. The usual methods of separation of these twocomponents are the use of another rectification column, cooling andphase separation or solvent extraction.

One of the conmercially important ways to manufacture n-propyl acetateis by the catalytic esterification of n-propanol with aectic acid.n-Propyl acetatc (b.p.=101.6° C.), n-propanol (b.p.=97.2° C.) and water(b.p.=100° C.) form a ternary azeotrope boiling at 82.2° C. andcontaining 59.5 weight percent n-propyl acetate, 19.5 wt. % n-propanoland 21 wt. % water. n-Propyl acetate also forms a binary azeotrope withn-propanol which boils at 94° C. and contains 37 wt. % n-propyl acetate,and a binary azeotrope with water boiling at 82.4° C. and containing 86wt. % n-propyl acetate. n-Propanol also forms a binary minimum azeotropewith water which boils at 88.1° C. and contains 71.8 wt. % n-propanol.Thus in the esterification of n-propanol with acetic acid to formn-propyl acetate and water, the rectification of this mixture has threebinary and a ternary azeotrope to contend with, and yields the lowestboiling constituent, namely the n-propyl acetate - n-propanol - waterternary azeotrope. It is therefore impossible to produce n-propylacetate from n-propanol and water mixtures by rectification because thelower boiling ternary azeotrope will always come off overhead as theinitial product. Any mixture of n-propyl acetate, n-propanol and watersubjected to rectification at one atmosphere pressure will produce anoverhead product boiling at 82.2° C. and containing 59.5 wt. % n-propylacetate, 19.5 wt. % n-propanol and 21 wt. % water. Extractivedistillation would be an attractive method of effecting the separationof n-propyl acetate from n-propanol if agents can be found that (1) willbreak the n-propyl acetate-n-propanol - water azeotrope and (2) are easyto recover from the n-propanol, that is, form no azeotrope withn-propanol and boil sufficiently above n-propanol to make the separationby rectification possible with only a few theoretical plates.

Extractive distillation typically requires the addition of an equalamount to twice as much extractive agent as the n-propylacetate-n-propanol - water on each plate of the rectification column.The extractive agent should be heated to about the same temperature asthe plate into which it is introduced. Thus extractive distillationimposes an additional heat requirement on the column as well as somewhatlarger plates. However this is less than the increase occasioned by theadditional agents required if the separation is done by azeotropicdistillation.

Another consideration in the selection of the extractive distillationagent is its recovery from the bottoms product. The usual method is byrectification in another column. In order to keep the cost of thisoperation to a minimum, an appreiable boiling point difference betweenthe compound being separated and the extractive agent is desirable. Itis also desirable that the extractive agent be miseible with n-propanolotherwise it will form a two-phase azeotrope with the n-propanol in therecovery column and some other method of separation will have to beemployed.

The breaking of this azeotrope by extractive distillation is a newconcept. The closest application of the concept might be the breaking ofthe methyl acetate - methanol described by Berg & Yeh, CHEMICALENGINEERING COMMUNICATIONS, p.3219-3223, 1984.

OBJECTIVE OF THE INVENTION

The object of this invention is to provide a process or method ofextractive distillation that will enhance the relative volatility ofn-propyl acetate from n-propanol in their separation in a rectificationcolumn. It is a further object of this invention to identify suitableextractive distillation agents which will eliminate the n-propylacetate - n-propanol - water ternary azeotrope and make possible theproduction of pure n-propyl acetate and n-propanol by rectification. Itis a further object of this invention to identify organic compoundswhich, in addition to the above constraints, are stable, can beseparated from n-propanol by rectification with relatively fewtheoretical plates and can be recycled to the extractive distillationcolumn and reused with little decomposition.

SUMMARY OF THE INVENTION

The objects of the invention are provided by a process for separatingn-propyl acetate from n-propanol which entails the use of certainoxygenated, nitrogenous and/or sulfur containing organic compounds asthe agent in extractive distillation.

DETAILED DESCRIPTION OF THE INVENTION

We have discovered that certain oxgenated and nitrogenous organiccompounds, some individually but principally as mixtures, willeffectively negate the n-propyl acetate - n-propanol - water ternaryazeotrope and permit the separation of pure n-propyl acetate fromn-propanol by rectification when employed as the agent in extractivedistillation. Table 1 lists the compounds, mixtures and approximateproportions that we have found to be effective. The data in Table 1 wasobtained in a vapor-liquid equilibrium still. In each case, the startingmaterial was the n-propyl acetate - n-propanol - water azeotrope. Theratios are the parts by weight of extractive agent used per part ofn-propyl acetate - n-propanol - water azeotrope. The relativevolatilities are listed for each of the two ratios employed. Thecompounds that are effective when used alone are triethanolamine andN-methylpyrrolidone. The compounds which are effective when used inmixtures of two compounds are ethylene glycol, 1,6-hexanediol andtetraethylene glycol. The two relative volatilities shown in Table 1correspond to the two different ratios employed. For example, in Table1, one part of triethanolamine with one part of of the n-propylacetate-n-propanol - water azeotrope gives a relative volatility of1.44, 6/5 parts of triethanolamine give 1.54. One half part ofN-methylpyrrolidone mixed with one half part of ethylene glycol with onepart of the n-propyl acetate - n-propanol - water azeotrope gives arelative volatility of 1.69, 3/5 parts of N-methylpyrrolidone plus 3/5parts of ethylene glycol gives 1.91. In every example in Table 1, thestarting material is the n-propyl acetate - n-propanol - water azeotropewhich possesses a relative volatility of 1.00.

                  TABLE 1    ______________________________________    Extractive Distillation Agents That Are Effective In    Separating n-Propyl Acetate From n-Propanol                                  Relative    Compounds         Ratios      Volatilities    ______________________________________    Triethanolamine   1       6/5.sup.                                      1.44  1.54    N--Methylpyrrolidone                      "       --      1.54    N--Methylpyrrolidone, Ethylene                      (1/2).sup.2                              (3/5).sup.2                                      1.69  1.91    glycol    N--Methylpyrrolidone,                      "       "       1.17  1.18    1,6-Hexanediol    N--Methylpyrrolidone, Tetra-                      "       "       1.24  1.16    ethylene glycol    N--Methylpyrrolidone,                      "       "       1.42  1.58    Triethanolamine    ______________________________________

                  TABLE 2    ______________________________________    Data From Runs Made In Rectification Column                Wt. % n-Propyl Acetate                               Relative    Agents        Overhead   Bottoms   Volatility    ______________________________________    Triethanolamine                  94.1       67.3      1.55    N--Methylpyrrolidone,                  91.3       44.1      1.78    Ethylene glycol 1:1    ______________________________________     Notes:     Compositions are reported on agent and waterfree basis     Agents added at 20 ml/min. & 50° C.     Ratio of mixed agents is by weight

Several of the compounds and mixtures listed in Table 1 and whoserelative volatility had been determined in the vapor-liquid equilibriumstill, were then evaluated in a glass perforated plate rectificationcolumn possessing 4.5 theoretical plates. The n-propylacetate-n-propanol - water mixture studied contained 59.5 wt. % n-propylacetate, 19.5 wt. % n-propanol and 21 wt. % water which is the ternaryazeotrope composition. In every case the ratio of n-propyl acetate ton-propanol in the overhead is greater than 3.05 and the results aretabulated in Table 2. Without the extractive agent, the overhead wouldbe the azeotrope whose ratio of n-propyl acetate to n-propanol is 3.05.This proves that the extractive agent is negating the azeotrope andmakes the rectification proceed as if the azeotrope no longer existedand brings the more volatile components, n-propyl acetate and water, outas overhead products. It is our belief that this is the first time thatthis has been accomplished for this azeotrope.

The data in Table 2 was obtained in the following manner. The charge was59.5% n-propyl acetate, 19.5% n-propanol and 21% water and after a halfhour of operation in the 4.5 theoretical plate column to establishequilibrium, triethanolamine at 50° C. and 20 ml/min. was pumped in. Therectification was continued for about two hours with sampling ofoverhead and bottoms after one hour, 1.5 hours and two hours. Theaverage of the three analyses is shown in Table 2 and was 94.1% n-propylacetate in the overhead and 67.3% in the bottoms, both on a water-freebasis which gives a relative volatility of 1.55 of n-propyl acetate ton-propanol.

This indicates that the ternary azeotrope has been negated andseparation accomplished. The n-propyl acetate comes off in the form ofits binary azeotrope with water which on condensation, immediately formstwo layers. The solubility of n-propyl acetate in liquid water is only1.6%.

THE USEFULNESS OF THE INVENTION

The usefulness or utility of this invention can be demonstrated byreferring to the data presented in Tables 1 and 2. All of the successfulextractive distillation agents show that n-propyl acetate, n-propanoland water can be separated from their ternary azeotrope by means ofdistillation in a rectification column and that the ease of separationas measured by relative volatility is considerable. Without theseextractive distillation agents, no improvement above the azeotropecomposition will occur in the rectification column. The data also showthat the most attractive agents will operate at a boilup rate low enoughto make this a useful and efficient method of recovering high purityn-propyl acetate from any mixture of these three including the ternaryminimum azeotrope. The stability of the compounds used and the boilingpoint difference is such that complete recovery and recycle isobtainable by a simple distillation and the amount required for make-upis small.

WORKING EXAMPLES EXAMPLE 1

The n-propyl acetate - n-propanol - water azeotrope is 59.5 wt. %n-propl acetate, 19.5 wt. % n-propanol and 21 wt % water. Fifty grams ofthe n-propyl acetate - n-propanol - water azeotrope and fifty grams oftriethanolamine were charged to an Othmer type glass vapor-liquidequilibrium still and refluxed for eleven hours. Analysis of the vaporand liquid by gas chromatography gave a vapor composition of 74.1%n-propyl acetate, 25.9% n-propanol; a liquid composition of 66.5%n-propyl acetate, 33.5% n-propanol. This indicates a relative volatilityof 1.44. Ten grams of triethanolamine were added and refluxing continuedfor another twelve hours. Analysis indicated a vapor composition of71.1% n-propyl acetate, 28.9% n-propanol; a liquid composition of 61.5%n-propyl acetate, 38.5% n-propanol which is a relative volatility of1.54.

EXAMPLE 2

Fifty grams of the n-propyl acetate - n-propanol - water azeotrope, 25grams of N-methylpyrrolidone and 25 grams of ethylene glycol werecharged to the vapor-liquid equilibrium still and refluxed for sevenhours. Analysis indicated a vapor composition of 73% n-propyl acetate,27% n-propanol; a liquid composition of 61.6% n-propyl acetate, 38.4%n-propanol which is a relative volatility of 1.69. Five grams ofN-methylpyrrolidone and five grams of ethylene glycol were added andrefluxing continued for another two hours. Analysis indicated a vaporcomposition of 73.5% n-propyl acetate, 26.5% n-propanol; a liquidcomposition of 59.2% n-propyl acetate, 40.8% n-propanol which is arelative volatility of 1.91.

EXAMPLE 3

A glass perforated plate rectification column was calibrated withethylbenzene and p-xylene which possesses a relative volatility of 1.06and found to have 4.5 theoretical plates. A solution comprising 268grams of n-propyl acetate, 88 grams of n-propanol and 94 grams of waterwas placed in the stillpot and heated. When refluxing began, anextractive agent consisting of pure triethanolamine was pumped into thecolumn at a rate of 20 ml/min. The temperature of the extractive agentas it entered the column was 50° C. After establishing the feed rate ofthe extractive agent, the heat input to the n-propyl acetate, n-propanoland water in the stillpot was adjusted to give a total reflux rate of10-20 ml/min. After one hour of operation, the overhead and bottomssamples of approximately two ml. were collected and analysed using gaschromatography. The overhead analysis was 94.1% n-propyl acetate, 5.9%n-propanol. The bottoms analysis was 67.3% n-propyl acetate, 32.7%n-propanol. Using these compositions in the Fenske equation, with thenumber of theoretical plates in the column being 4.5, gave an averagerelative volatility of 1.57 for each theoretical plate. After 11/2 hoursof total operating time, the overhead and bottoms samples were againtaken and analysed. The overhead composition was 95.5% n-propyl acetate,4.5% n-propanol and the bottoms composition was 69% n-propyl acetate,31% n-propanol. This gave an average relative volatility of 1.58 foreach theoretical plate. After two hours of total operating time, theoverhead and bottoms samples were again taken and analysed. The overheadcomposition was 95.7% n-propyl acetate, 4.3% n-propanol and the bottomscomposition was 74.8% n-propyl acetate, 25.2% n-propanol. This gave anaverage relative volatility of 1.57 for each theoretical plate.

EXAMPLE 4

A solution of 268 grams of n-propyl acetate, 88 grams of n-propanol and94 grams of water was placed in the stillpot of the same column used inExample 3 and heat applied. When refluxing began, an extractive agentcomprising 50% N-methylpyrrolidone and 50% ethylene glycol was fed intothe top of the column at a feed rate of 20 ml/min. and a temperature of50° C. After establishing the feed rate of the extractive agent, theheat input to the n-propyl acetate, n-propanol and water in the stillpotwas adjusted to give a total reflux rate of 10-20 ml/min. Havingestablished the reflux rate, the column was allowed to operate for onehour. After one hour of steady operation, overhead and bottoms samplesof approximately two ml. were collected and analysed using gaschromatography. The overhead composition was 91.3% n-propyl acetate,8.7% n-propanol, the bottoms composition was 44.1% n-propyl acetate,55.9% n-propanol. Using these compositions in the Fenske equation withthe number of theoretical plates in the column being 4.5, gave anaverage relative volatility of 1.78 for each theoretical plate. After11/2 hours of total operation, the overhead composition was 92% n-propylacetate, 8% n-propanol and the bottoms composition was 47.5% n-propylacetate, 52.5% n-propanol. This gave an average relative volatility of1.76 for each theoretical plate. After two hours of total operation, theoverhead composition was 92.2% n-propyl acetate, 7.8% n-propanol and thebottoms composition was 46.5% n-propyl acetate, 53.5% n-propanol. Thisgave an average relative volatility of 1.79 for each theoretical plate.

The nature of the present invention having been described, what we wishto claim as new and useful and secure by Letters Patent is:
 1. A methodfor recovering n-propyl acetate from a mixture of n-propyl acetate,n-propanol and water which comprises distilling a mixture of n-propylacetate, n-propanol and water in a rectification column in the presenceof about one part of extractive agent per part of n-propyl acetate -n-propanol - water mixture, recovering n-propyl acetate and water asoverhead product and obtaining the extractive agent and n-propanol fromthe stillpot, the extractive agent comprises at leastN-methylpyrrolidone.
 2. The method of claim 1 in which the extractiveagent comprises a mixture of N-methylpyrrolidone and at least onematerial from the group consisting of ethylene glycol, 1,6-hexanediol,tetraethylene glycol and triethanolamine.